I am have installed 4GB RAM into my Vostro and just discovered it is 600 MBish short because 32bit Vista Business cannot address it. I
It´s not only a matter of OS but also a matter of Chipset/Bios.
Many of the Chipset which support 4GB only let you use an smaller amount. For example, i915G gives you only ~3,25GB maximum, even if you have 4GB plugged in physically